The present invention relates to radio receivers and transmitters, and, more particularly, to a spread spectrum radio system in which an intermediate frequency is transmitted along with data and used at the receiver to control a local oscillator.
Radio systems using spread spectrum modulation techniques are becoming increasingly popular for a variety of communication applications. In a spread spectrum system, the transmitted signals spread over a frequency band that is wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby allowing such systems to operate outside of certain FCC licensing requirements. Given these advantages, such communication systems are becoming desirable for commercial data transmission.
A common type of such communication system is a direct sequence spread spectrum modulation system. In direct sequence spread spectrum, an RF carrier is modulated by a digital code sequence having a bit rate much higher than that of the information signal. Typically, the carrier is modulated by two data streams in quadrature. Each stream includes one phase when the data code sequence represents a logic one and a 180.degree. phase shift when the data stream code sequence represents a logic 0. Since the digital code sequence includes square-wave half periods that vary in duration, the spectral power envelope of a direct sequence modulated signal is represented by [(SIN x)/x].sup.2. This quadrature modulation is commonly referred to as quadrature phase shift key (QPSK) modulation. Bi-polar phase shift key modulation may also be used.
During operation, the receiver recovers the signal using two separate processes. First, the receive signal is down converted from the center frequency (f.sub.c) of the signal's carrier to a fixed intermediate frequency to enable further processing of the signal. Conventional signal processing techniques, such as those used in heterodyne radios, can be applied to down convert the received signal. Next, the spreading code modulation is removed or demodulated to reveal the information carried in the signal. Demodulating the spread signal is accomplished by multiplying the signal by a spreading code sequence identical in structure and synchronization in time with the received signal. This process is known as correlation. Down converting and demodulating the spread signal may be performed simultaneously or in successive stages.
In conventional heterodyne receivers, the received signal is mixed against a sine wave generated by a local oscillator having a frequency different from the center frequency of the carrier (f.sub.o). The mixer generates sum and difference frequencies, which correspond to the originally received signal. Basically, the mixer performs a frequency conversion resulting in the received signal being converted to a replica of the received signal at an intermediate frequency (IF) comprising the difference between the carrier frequency and the local oscillator frequency (f.sub.c -f.sub.o). The information can then be demodulated at a fixed frequency in the IF stage of the receiver.
One drawback of a heterodyne receiver is that demodulation at the intermediate frequency requires additional translation mixers and tuned filters to attenuate the various noise signals that result from the heterodyne down conversion, thereby adding unnecessary complexity to the receiver circuitry. In addition, the frequency conversion process often allows undesired signals, known as the image frequency, to pass through into the IF signal processing stage. Thus, an important consideration in heterodyne receiver design is rejection of the image frequency components.
Another problem relates to heterodyne systems designed to both receive and transmit signals. To transmit a signal having the same carrier frequency of the received signal, an oscillator is required to provide the carrier frequency for the receiving and transmitting system. Since the local oscillator of the heterodyne receiver produces a frequency offset from the carrier frequency, either the local oscillator must be retuned for transmission operation or a second oscillator must be provided. Rapid returning of the local oscillator is problematic at relatively high transaction rates, and can result in transmission delay. Also, the addition of the second oscillator further increases the complexity and cost of the radio system.
Yet another problem with such systems relates to the requirement of highly stable and precise local oscillators and related components to ensure that the local oscillator in one radio is virtually identical to the local oscillator in another radio. These systems are substantially intolerant to changes in another radio's local oscillator frequency as well as changes in the frequency after transmission due to various signal altering effects caused by the transmission environment. For example, there are many existing and proposed systems using a low-orbit satellite or network of satellites to provide communications between satellites and between satellites and ground-based stations. Examples of these systems are global pagers and telephone systems. The satellites used in these systems are preferably low-earth orbiting satellites (LEO's), which travel at a relatively high and varying speed with respect to any station or satellite with which it communicates. These relative changes in speed subject the transmission frequency to the Doppler effect.
The Doppler effect on an LEO's frequency is quite pronounced and, without a means of frequency correction, can cause severe transmission errors or a complete lack of communications ability. If an LEO is orbiting 1,000 kilometers above the earth's surface and the orbital period at the earth's surface is 155.4 minutes, the satellite would be traveling at 1,103 meters per second. If the transmission frequency of the satellite is 1 gigahertz, the measured frequency at a location on earth in the path of the satellite would vary .+-.37 kilohertz. If the satellite is approaching both the up-link transmitting location as well as the receiving location, the signal frequency from the LEO may change as much as .+-.74 kilohertz. These frequency changes do not permit the use of prior art spectrum receivers, non-spread or spread, without using complex frequency correction methods, which are expensive to employ.
Accordingly, a radio receiver and transmitter avoiding the complexity and drawbacks of heterodyne reception is needed. Furthermore, there is a need for an economical radio system capable of tracking a transmitted signal without regard to frequency changes due to phenomena, such as the Doppler effect, or drift in the receiving radio or transmitting radio's transmission frequencies.